A continuing goal in integrated circuitry design is to make ever denser, and therefore smaller, circuit devices. This results in thinner layers and smaller geometries. Further, new deposition techniques and materials are constantly being developed to enable circuit devices to be made smaller than the previous generation.
One common circuitry device is a field effect transistor. Such includes a pair of conductive source/drain regions having a semiconductive channel region received therebetween. A conductive gate is received proximate the channel region, with a gate dielectric layer being received between the gate and the channel region. Application of a suitable voltage potential to the gate enables current to flow between the source/drain regions, with the transistor being capable of essentially functioning as a switch.
A common material for the gate dielectric layer is silicon dioxide. However as device geometry continues to shrink, the thinness of the gate dielectric layer will likely preclude silicon dioxide from being used as a gate dielectric material due to its too small of a dielectric constant. Accordingly, other dielectric materials with higher dielectric constants have been proposed for use as gate dielectric layers. Exemplary such materials include Al2O3 (typically having a dielectric constant of about nine), and HfO2 (typically having a dielectric constant ranging from about 18 to about 25).
Even high dielectric constant materials will most likely be deposited to very thin thicknesses (for example 10 Angstroms or less) to achieve desired circuit densities. One comparatively new technique for depositing very thin layers is atomic layer deposition (ALD). Described in summary, ALD includes exposing an initial substrate to a first gaseous precursor-chemical species to accomplish chemisorption of the species onto the substrate. Ideally, the chemisorption would form a monolayer that is uniformly one atom or molecule thick on the entire exposed initial substrate. Alternately considered and desirably, a saturated monolayer is formed. However more typically, chemisorption does not occur on all portions of the substrate such that the monolayer is discontinuously formed thereover. Nevertheless, such a monolayer is still considered a monolayer in the context of this document.
The first chemical precursor is purged from the substrate and a second gaseous precursor is provided to enable a second species to chemisorb onto and with the first monolayer. Any remaining of the second gaseous precursor is then purged and the steps are repeated, with exposure of the second species monolayer to the first gaseous precursor. In some instances, the two monolayers may be of the same species. Also, a third species or more from additional gaseous precursors may be successively chemisorbed and purged, as described for the first and second precursors. Further, it is noted that one or more of the first, second and third precursors can be mixed with an inert gas to speed up pressure saturation within a reaction chamber. Purging may involve a variety of techniques including, but not limited to, contacting the substrate and/or monolayer with a carrier gas and/or lowering pressure.
ALD is often described as a self-limiting process in that a finite number of sites exist on a substrate to which the first species may form chemical bonds. The second species might only bond to the first species, and thus may also be self-limiting. Once all of the finite number of cites on a substrate are bonded with the first species, the first species will often not bond to other of the first species already bonded with the substrate.
One common semiconductive channel material is silicon, for example monocrystalline and/or polycrystalline silicon. When forming HfO2 or Al2O3 by ALD, the second gaseous precursor will typically contain oxygen, for example O2, O3 and/or H2O. In many instances, exposure of a silicon channel to a first species containing hafnium or aluminum will not completely saturate (meaning cover) all of the exposed silicon surface. Accordingly, at the conclusion of a purge cycle, some silicon will still typically be outwardly exposed. Typically, exposure of this exposed silicon to an oxygen containing precursor will undesirably form silicon dioxide from the exposed substrate material, as well as create hafnium oxide or aluminum oxide from the discontinuously adhered monolayer. The result can be the formation of undesired silicon dioxide in combination with the hafnium oxide and/or aluminum oxide. This can result in an unacceptable lowering of the dielectric constant of the layer being formed, precluding its use in very thin gate dielectric layers.
While the invention was motivated in addressing the above identified issues, it is in no way so limited. The invention is only by the accompanying claims as literally worded, without interpretative or other limiting reference to the specification, and in accordance with the doctrine of equivalents.